The present invention relates generally to circuits and methods that function as constant current sources. More particularly, this invention pertains to methods and circuits for increasing stability of constant current sources in low current operating conditions.
Referring to FIG. 1, a conventional constant current source controller 104 (micro-controller uC) typically needs two signals to maintain a predetermined current level, a current sensing signal (I_sense) and a reference current signal (I_ref_input). A current source tank 102 of the constant current source 100 may be frequency controlled, duty cycle controlled, or other type of current source tank with a control input that is used to adjust the output current of the current source tank 102. A current sensing resistor R_I_sense is in series with a load R_load (e.g., a light source). It is desirable to minimize the resistance value of the current sensing resistor R_I_sense to minimize power consumption by the current sensing resistor R_I_sense. For a wide range of controlled operating output current, the voltage across the current sensing resistor R_I_sense will also be very wide. For example, for an output current range from 1.4 A to 10 mA, the current sensing signal (i.e., voltage) across the current sensing resistor R_I_sense will vary from 0.14V to 0.001V if the resistance of the current sensing resistor R_I_sense is chosen to be 0.1 ohm, a relatively low resistance value to minimize power consumption. Because this voltage signal is relatively small, a current sensing amplifier such as operational amplifier (OPAMP) U1 is used to amplify the signal. A first resistor R1 and a second resistor R2 determine the gain ratio of the OPAMP U1. The gain between the voltage across the current sensing resistor (i.e., I_sense_in) and the current sensing signal I_sense received at the controller 104 is defined in Equation 1.
                              I          sense                =                                                            R                ⁢                                                                  ⁢                1                            +                              R                ⁢                                                                  ⁢                2                                                    R              ⁢                                                          ⁢              2                                ·                      I            sense_in                                              EQUATION        ⁢                                  ⁢        1            
The controller 104 compares the current reference signal (I_ref_input) and current sensing signal I_sense (i.e., current feedback signal) and maintains the output current level set by current reference signal I_ref_input. The control target is given in Equation 2.
                              I          ref_input                =                              I            sense                    =                                                                      R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                            R                ⁢                                                                  ⁢                2                                      ·                          I              sense_in                                                          EQUATION        ⁢                                  ⁢        2            
The relationship between the reference current signal I_ref_input and the voltage across the current sensing resistor R_I_sense (i.e., I_sense_in) is shown in Equation 3.
                                          I            ref_input                                I            sense_in                          =                                            R              ⁢                                                          ⁢              1                        +                          R              ⁢                                                          ⁢              2                                            R            ⁢                                                  ⁢            2                                              EQUATION        ⁢                                  ⁢        3            
The power supply voltage or bias voltage V1 of the controller 104 is typically relatively low (e.g., 3V to 5V). The current sensing signal I_sense received at the controller 104 must be less than the supply voltage V1 of the controller 104 to prevent lockup or instable operation of the controller 104. If the output current range is 100% to 1%, the current sensing signal I_sense received at the controller 104 and used by the controller 104 to control operation for the current source tank 102 would be 5V at full output to 50 mV at minimum output. A 50 mV signal is extremely small for the controller 104 to accurately sense. Circuit ground noise may be greater than 50 mV. This low voltage current feedback signal could be problematic for the controller 104 to accurately determine the output current of the current source tank 102 and may result in stability problems of the circuit.